The utilization of waste heat to preheat water supplied to a dishwasher is known, but has not been accomplished in the most effective manner. Thus, U.S. Pat. No. 3,789,860 discloses a dishwasher, the normal operation of which produces steam passing into a room in which the dishwasher is installed. Instead, the steam is collected in a hood and passed through a heat exchanger to preheat incoming water which along with the condensate from the steam, is supplied to a pre-rinse stage of the dishwasher. However, the hot water supplied for rinsing by a hot water heater, or similar source, is not affected, so that none of the recovered energy is utilized to reduce the fuel consumed by the hot water heater. U.S. Pat. No. 3,946,802 discloses a method of and apparatus for recovery of heat involving a supply pipe for supplying cold water to a water heater and a waste pipe which is supplied with waste water from different consumption points, including dishing machines, washing machines bath tubs, kitchen sewage and the like. Such waste water is directed to a collecting receptacle from which the waste water is either bypassed to a second receptacle or supplied to a concurrent flow heat exchanger in which the heat is inefficiently utilized, since the hottest waste water first transfers heat to the coldest incoming water. Temperature responsive elements control the bypass flow, so that if the waste water in the receptacle is at a lower temperature than the incoming water reaching the concurrent heat exchanger, the waste water will be bypassed to a second receptacle. There is a probability that this system will delay unduly in responding to a new supply of waste water, since the collecting receptacle has a central overflow and considerable waste water could be bypassed before the temperature sensor responds. The waste water bypassed to the second receptacle is, in turn, bypassed to the sewer or directed to a second concurrent flow heat exchanger, with the same loss of efficiency and probability of undue delay in response to incoming waste water.
The apparatus of the invention, for conserving energy for a device using a heated liquid from a liquid heater and producing waste liquid at a lower temperature than the heated liquid, may include a holding tank for collecting and releasing the waste liquid, an outlet adjacent the holding tank, an upright baffle separating the holding tank from the outlet, whereby waste liquid from the holding tank may overflow into the outlet, equipment including a conduit for transferring waste liquid from the holding tank to a heat exchanger to preheat incoming liquid for use in the device after heating in the liquid heater, equipment including a conduit for transferring the preheated liquid to the liquid heater for heating the preheated liquid to a temperature sufficient for use in the device and equipment including a conduit for transferring waste liquid from the heat exchanger to the outlet. The apparatus may further include a device, such as a dishwasher, using heated water from a hot water heater, a countercurrent heat exchanger for preheating the feed water to the hot water heater, a separate tank for collecting waste water discharged from the device and a pump for supplying the waste water to the heat exchanger at essentially the same time that the device requires hot water from the heater, as for rinsing. Thus, the incoming cold feed water is preheated at the time it is needed by the hot water heater. A dishwasher is ideally adapted for this usage, although the flow of collected waste water through the heat exchanger may be timed to coincide with hot water requirements of the device, or even another device operated in conjunction therewith in the case of devices other than a dishwasher. For a dishwasher, used waste water may be collected in a holding tank which is provided with a removable tray and a fixed screen for straining unwanted food or the like from the waste water. When the waste water is sufficiently electrically conductive, the holding tank may be provided with a series of electrodes, which turn the motor off when a low level of waste water in the holding tank is reached, turn the pump on when a higher level of waste water in the holding tank is reached and actuate an alarm, such as a light, buzzer, bell, etc., when an unduly high level of waste water in the tank is reached. The holding tank may instead be provided with a mechanical level sensor, such as a pivoted float mounted on a stem which actuates a switch for turning the pump motor on and off.
The heat exchanger is preferably of the type produced in accordance with my copending application Ser. No. 20,615 filed Mar. 15, 1979 now U.S. Pat. No. 4,451,960 entitled "Method of Producing Multiple Coil, Multiple Tube Heat Exchanger." As disclosed therein, each multiple tube row is wound outside the preceding row and the tubes for the respective fluid alternate, so that a tube carrying one fluid and passing in one direction with respect to the preferably hollow axis of the coil is interspaced in its own row with tubes carrying the other fluid in the opposite direction, as well as essentially alternating with tubes in the rows inside and outside of the aforesaid row. In addition, a heat conductive mastic is compressed between the tubes as they are formed into the spiral, generally circular rows.
The heat exchanger may be mounted at a position adjacent to or remote from the dishwasher, such as adjacent the hot water heater. The holding tank may be constructed so as to be beneath the dishwasher, or the holding tank and heat exchanger may be incorporated as a single unit, which may conveniently be mounted adjacent the dishwasher, such as beneath the dish table.